1. Field of the Invention
The present invention relates to improvements in methods for treating fibrous lignocellulose-containing biomass prior to refining the biomass. The purpose of the invention is to increase the digestibility and chemical reactivity of the substrate resulting from the treatment. The invention is particularly directed to the use of high-shear forces to destroy the fibrous structure of the biomass and render the cellulose more accessible for hydrolysis.
2. Description of the Related Art
Lignocellulose-containing material can be refined to sugars, protein, and lignin. While protein and lignin extraction methods have been effective, at commercially viable enzyme loadings, sugar yields historically are well below the theoretical level. This is thought to be due to the close physical and chemical association between lignin and cellulose and hemicellulose within the cell wall of woody plants. This close association or bonding reduces the availability of the cellulose for hydrolysis. Additionally, when cellulose molecules exhibit a high degree of crystallinity in their structure they are even more resistant to hydrolysis. As a result, a great deal of effort has been directed toward overcoming these problems and thereby increasing sugar yields. Most of this effort has been directed toward finding methods for the removal of lignin and hemicellulose and the disruption and destruction of the crystalline structure of the cellulose molecule.
A key to increasing the sugar yields from lignocellulose-containing biomass is to increase access to cellulose and hemicellulose by the enzymes or other chemical or biological agents used to hydrolyze the cellulose into sugar. Thus, attempts have been made to destroy the fibrous structure of the biomass and thereby increase the reactive surface area of the resulting substrate. The greater the reactive surface area of the substrate, i.e., the treated biomass, the more access that the enzymes or other agents have to the cellulose in the substrate.
Many different techniques have been used to treat lignocellulosic biomass specifically to increase the reactive surface area of the resulting substrate. These techniques have resulted in varying degrees of effectiveness.
Concentrated acid has been used to chemically hydrolyze fibrous biomass. Biomass and the acid are combined, forming a broth. The broth is held in a vat at ambient temperature for a period of time sufficient to break down the biomass and hydrolyze the cellulose, hemicellulose, proteins and lignin. While effectively breaking down the biomass and hydrolyzing the cellulose and hemicellulose, this process creates the difficult problem of separating the sugars from the acid. Presently, there is no efficient and economical way to separate the sugars and acid, making this process undesirable for a commercial application. Furthermore, concentrated acid is corrosive and requires extreme care in handling.
Dilute acid has been used in a process to promote the disintegration of fibrous biomass. Biomass and dilute acid are combined, forming a broth. The broth is "cooked" at high temperature causing the hemicellulose to hydrolyze. The acid must then be neutralized and washed out of the mix. Following the removal of the acid, the remaining biomass is treated with high enzyme loadings, in excess of 20 IU's/gram of substrate, to hydrolyze the cellulosic fraction.
Washing the acid out of the mix creates a waste stream that must be treated prior to discharge from the processing facility. "Cooking" at high temperature causes formation of furfural and hydroxymethyl furfural in the sugar component of the mix. Furfural and hydroxymethyl furfural are toxic and inhibit fermentation, therefore they must be separated from the desirable sugars. Steps have been taken to minimize furfural production through a two-stage/two-temperature approach. First, a lower temperature hydrolyzes the C.sub.5 sugars, which are removed. Then the remaining fiber is subjected to higher temperatures for decrystallization and delignification.
Separating the acids and other toxic products from the desirable sugars is difficult and expensive, limiting the usefulness of this process. The use of acids also raises environmental issues because they are considered pollutants.
Steam has been used to disintegrate and defiberize biomass. This is done under high pressure and at a high temperature exploding the fibers within the biomass. Because this procedure must be done at high temperatures, degradation products such as fuurfural and hydroxymethyl furfural are created. These degradation products are toxic and will inhibit fermentation even at very low levels, therefore they must be removed prior to any subsequent fermentation. The water wash streams used to remove the degradation products become toxic themselves and must be treated before they can be discharged from the processing facility. Besides the environmental cost, the use of steam is extremely costly because of the energy that is required which is not recoverable, and because of the equipment costs that are required to practice this technique.
Ammonia has also been used to explode and disintegrate fibrous biomass. This technique, known as AFEX for Ammonia Freeze Explosion, is performed under high pressure. The pressure and temperature required are dependent upon the substrate being treated. The process is generally performed at temperatures from 50.degree.-90.degree. C. This process does not degrade sugars and suffers from few, if any, adverse environmental effects. Nonetheless, the ammonia must be reclaimed and this entails certain costs. Further details of this technique are described in U.S. Pat. No. 5,037,663 issued to Bruce E. Dale, the subject matter of which is incorporated herein by reference.
Grinding methods have been tried to essentially chop the biomass into pieces small enough for effective sugar hydrolysis. These attempts have not resulted in commercially acceptable sugar yields even though the biomass has been ground to particle sizes as small as 37 microns This technique requires no chemicals, so chemical recovery is not a problem. The drawback to grinding is that it is very energy intensive, inefficient, and an expensive means of disintegrating fibrous biomass. Most grinding methods essentially cut the end off of the fiber bundle time after time. The fiber bundle is strongest perpendicular to the fiber axis, and it is in this perpendicular direction that most of the cutting is done. Moreover, there is a tendency to compress the biomass structure crushing the cracked and broken surfaces making them less susceptible to penetration by water and enzymes or chemicals. This compression effect is even more significant in biomass with higher lignin content due to the naturally recalcitrant, i.e. highly crystallized, nature of the lignin.
To achieve very small particle sizes, the biomass must be ground repeatedly. Consequently, such grinding consumes a great deal of energy making grinding simply uneconomical as a means of treatment. The cost of grinding is prohibitive based on the energy consumption alone, not taking into account equipment costs, including the cost of repair and general wear of the machinery.
Strong alkali agents have been used in conjunction with shear forces produced by an extruder device to both chemically and physically disintegrate fibrous biomass. This technique uses a mixture of a strong alkali and a peroxide combined with biomass. The technique requires solids loadings in excess of 30% solids. The alkali-peroxide/biomass broth is exposed to shear forces in an extruder device. An extruder forces the biomass through a pipe of decreasing diameter under great pressure and finally through a small orifice where there is a substantial pressure release. The extruder device has a masticatory effect on the biomass, grinding and chewing the particles. The extruder is particularly used for its mixing capabilities, i.e., dispersing the biomass in the alkali-peroxide mix. While producing high-shear forces, an extruder actually functions similar to a grinder. The resulting broth is then held in residence for up to 24 hours to complete the process.
This method requires the use of potentially toxic chemicals. It also may require extended treatment times, requiring that the resulting broth be held in residence for up to 24 hours. Recovery of the alkali and peroxide is not necessarily required, but special care must be used in handling these toxic chemicals. This method accordingly is expensive due to high energy costs and toxic chemical handling costs, and can be inefficient due to the time required for treatment. Further details of this technique are described in U.S. Pat. No. 4,997,488 issued to John M. Gould and Brian J. Jasberg, the subject matter of which is incorporated herein by reference.
High-frequency, rotor-stator devices have been used to aid in disintegrating the starch component of certain agriculture products like corn and tubers prior to refining the starch to alcohol. Corn and other starch-containing materials, however, have little or no lignin associated with their cell structure; and their cell structure is minimally fibrous when compared with a fibrous lignocellulosic biomass. Consequently, while starch-containing materials have been subjected to mechanical breakdown by the use of rotor-stator devices, it has been generally considered that these devices would not be effective in breaking down lignocellulose-containing materials. Moreover, the fibrous component of these materials has not been hydrolyzed and the accumulation of the fibrous component in the equipment has caused equipment failures.
Hardwood having a lignin content from 20-23% has been subjected to high-frequency, rotor-stator devices as an adjunct to furfural production. In these instances, the wood was first ground and the particles "cooked" with dilute acid at high temperature before use of high-frequency, rotor-stator devices. A rotor-stator device, known as a conical tool marketed under the trade name SUPRATON by Krupp Industrietechnik GmbH, was used primarily for its grinding and dispersion capability, i.e., to disperse the ground wood particles in the high temperature, acid broth. This process has the same drawbacks as the dilute acid process because it requires separation of the acid and toxic by-products from the desirable sugars. The process also creates a toxic waste stream resulting from the separation of the sugars from the mix. Furthermore, the effectiveness of this method was minimal.